WJEC Physics for AS: Study and Rev Guide

Q & A 1 Seren’s answer (a) (i) The force exerted multiplied by the distance 3 travelled in the direction of the force. 3 (ii) [F] = N = kg m s - 2 [d] = m 3 \ [W] = J = [F] × [d] = kg m 2 s - 2 3 (b) (i) D gpe = mg D h = 70 × 9.81 × (120 sin 20) 3 = 28183.82739 J = 28.2 kJ (3 s.f.) 3 (ii) D KE = æ ö æ ö - ç ÷ ç ÷ è ø è ø 2 2 2 1 1 1 2 2 mv mv 3 = æ ö æ ö ´ - ´ ç ÷ ç ÷ è ø è ø 2 2 1 1 70 21 70 6 2 2 3 = 14175 J = 14.2 kJ 3 (c) (i) Energy can neither be created nor destroyed, but converted into other forms of energy. 3 (ii) The gravitational potential energy that has been lost between A and B has been converted into other forms of energy, 3 including thermal, kinetic and sound. (d) Fx = - 2 2 1 1 2 2 mv mu 3 120 F = ´ - ´ 2 2 1 1 70 21 70 6 2 2 7 120 F = 14175 3 F = 118 N (3 s.f.) 7 Examiner commentary (a) (i) Seren has correctly included the often- forgotten statement about direction. (ii) The first mark is for correctly identifying the force and distance units and the second for the manipulation. Note: Seren has used square brackets, [. . .], to indicate ‘the unit of’, which is quick. (b) (i) Seren has gained 1 mark for correct substitution and 1 mark for the answer. In this case, converting to kJ and expressing to 3 s.f. gained no extra credit. (ii) Fully correct. Note that, if Seren had slipped up in the final calculation, she would have received 2 marks because her working was clear and correct. (c) (i) A mark which most candidates achieve. (ii) Seren has identified that the initial GPE was converted into several other forms [i.e. not just kinetic] but has missed out any discussion of mechanism, e.g. work done against friction/drag. (d) Seren has used the principle: Work done = energy transfer and used it to calculate a force from the change of KE. Unfortunately, she has calculate the resultant force on the skier, not the resistive force. Seren gains 13 out of 16 marks. AS Physics: Study and Revision Guide 168 (a) (i) Define work. [2] (ii) Hence express the unit of work, J, in terms of the SI base units kg, m and s. [2] (b) 120m A B 20° A skier of mass 70 kg descends a slope as shown. The skier passes point A at a speed of 6 m s - 1 and B at 21 m s - 1 . Calculate for the descent from A to B : (i) The gravitational potential energy lost by the skier. [2] (ii) The kinetic energy gained by the skier. [3] (c) (i) State the principle of conservation of energy. [1] (ii) Discuss your answer to (b) (i) and (ii) in terms of this principle. [2] (d) Calculate the mean resistive force experienced by the skier between A and B. [4]